Does the Badal optometer stimulate accommodation accurately?

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Mesure de la biréfringence optique des fibres textiles. 1re partie: Démonstration théorique

En este trabajo se desarrollan los aspectos teóricos de la medida de la birrefringencia en cuerpos anisotrópicos por el método de Nikitin-Berek. También se desarrolla su aplicación específica a medidas de la birrefringencia en las fibras textiles en un microscopio de polarización con la ayuda de un compensador Berek. La importancia de la puesta a punto de esta técnica está relacionada con la medida indirecta de la orientación de las fibras a partir de valores de birrefringencia de manera que, cuanto mayor es la birrefringencia, mayor es la orientación de las macromoléculas en la dirección del eje de la fibra.This work sets out to examine the theoretical aspects of measuring birefringence in anisotropic bodies using the Nikitin-Berek method. This method is also used to develop a specific application for measuring birefringence in textile fibres under a polarizing microscope with the help of a Berek compensator. The importance of fine-tuning this technique is related to the indirect measurement of fibre direction based on birefringence values in such a way that the higher the birefringence, the higher the orientation of the macromolecules in the direction of the fibre’s axis.Les travaux ont consisté à développer les aspects théoriques de la mesure de la biréfringence pour des corps anisotropiques selon la méthode de Nikitin-Berek. L’application spécifique aux mesures de la biréfringence des fibres textiles est également développée avec un microscope polarisant équipé d’un compensateur Berek. L’importance de la mise au point de cette technique est liée à la mesure indirecte de l’orientation des fibres à partir de valeurs de biréfringence de sorte que plus la biréfringence est importante, plus l’orientation des macromolécules dans l’axe de la fibre l’est aussi

Effect of Experimental Conditions in the Accommodation Response in Myopia

SIGNIFICANCE The accommodative response is more affected by the type of refractive error than the method of stimulation, field of view (FOV), or stimulus depth. PURPOSE This study aims to analyze the effect of stimulation method, stimulus depth, and FOV on the accommodation response (AR) for emmetropes (EMM), late-onset myopes (LOM), and early-onset myopes (EOM). METHODS Monocular AR was measured in 26 young observers (n = 9 EMM, n = 8 LOM, n = 9 EOM) under 60 different viewing conditions that were the result of permuting the following factors: (1) stimulation method (free space or Badal lens viewing), (2) stimulus depth (flat or volumetric), (3) FOV (2.5, 4, 8, 10, and 30°), and (4) accommodative stimulus (AS: 0.17, 2.50, and 5.00 diopters [D]). RESULTS Mixed analysis of variance for 2.50 D of AS resulted in a significant effect of refractive group (F = 6.77, P < .01) and FOV (F = 1.26, P = .04). There was also a significant interaction between stimulus depth and FOV (F = 2.73, P = .03) and among stimulation method, FOV, and refractive group (F = 2.42, P = .02). For AS of 5.00 D, there was a significant effect of refractive group (F = 13.88, P < .01) and stimulation method (F = 5.16, P = .03). There was also a significant interaction of stimulation method, stimulus depth, and refractive group (F = 4.08, P = .03). When controlling for all interactions, LOM showed larger lags than EMM and EOM; the AR did not significantly change for fields of 8, 10, and 30°, and it did not significantly differ for different stimulation methods or stimulus depth. CONCLUSIONS Previously reported differences in AR when using lens-based methods compared with free space viewing may be explained by the effect of other factors such as the FOV or the depth of the stimulus. Targets with an FOV of 8 or 10° may be optimal for accurate ARs.Preprin

Random Changes of Accommodation Stimuli: An Automated Extension of the Flippers Accommodative Facility Test

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Speckle reduction in double-pass retinal images using variable-focus lenses

Speckle arises in double-pass images when coherent light is scattered by the retina. Since this noise degrades the images that are used to characterize the eye, there is special attention in reducing speckle when working with instruments based on retina reflections. In this work, we present a method for speckle reduction in double-pass retinal images by producing minor periodic variations in the vergence of the beam entering the eye with a variable-focus lens during image recording. Measurements in an artificial and a real eye following the implementation of the method corroborate the speckle reduction.Postprint (published version

Stimulus Unpredictability in Time, Magnitude, and Direction on Accommodation

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Response for light scattered in the ocular fundus from double-pass and Hartmann–Shack estimations

Double-pass (DP) and Hartmann--Shack (HS) are complementary techniques based on reflections of light in the ocular fundus that may be used to estimate the optical properties of the human eye. Under conventional data processing, both of these assessment modes provide information on aberrations. In addition, DP data contain the effects of scattering. In the ocular fundus, this phenomenon may arise from the interaction of light with not only the retina, but also deeper layers up to which certain wavelengths may penetrate. In this work, we estimate the response of the ocular fundus to incident light by fitting the deviations between DP and HS estimations using an exponential model. In measurements with negligible intraocular scattering, such differences may be related to the lateral spreading of light that occurs in the ocular fundus due to the diffusive properties of the media at the working wavelength. The proposed model was applied in young healthy eyes to evaluate the performance of scattering in such a population. Besides giving a parameter with information on the ocular fundus, the model contributes to the understanding of the differences between DP and HS estimations.Postprint (author's final draft

Tear film stability assessment by corneal reflex image degradation

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Effect of apparent depth cues on accommodation in a Badal optometer

Background: The aim was to analyse the effect of peripheral depth cues on accommodation in Badal optometers. Methods: Monocular refractions at 0.17 and 5.00 D of accommodative stimulus were measured with the PowerRef II autorefractor (Plusoptix Inc., Atlanta, Georgia, USA). Subjects looked (randomly) at four different scenes, one real scene comprising familiar objects at different depth planes (Real) and three virtual scenes comprising different two-dimensional pictures seen through a Badal lens. The first image consisted of a photograph of the real scene taken in conditions that closely mimic a healthy standard human eye performance (out-of-focus [OoF] blur); the second image was the same photograph rendered with a depth of focus to infinity (OoF sharpness); and finally the third image consisted of a fixation target and a even white surrounding (White). In all cases the field of view was 25.0° and the fixation target was a Maltese cross subtending to two degrees. Results: Twenty-eight right eyes from healthy young subjects were measured. The achieved statistical power was 0.9. At 5.00 D of accommodative stimulus, the repeated measures analysis of variance was statistically significant (p¿<¿0.05) and the corresponding Bonferroni post hoc tests showed the following mean accommodative response differences and standard deviation (p-value) between the real and the virtual scenes: real-white¿=-0.66¿±¿0.92 D (p¿<¿0.01); real-OoF sharpness¿=¿-0.43¿±¿0.88 D (p¿=¿0.07); real-OoF blur¿=-0.25¿±¿0.93 D (p¿=¿0.89). Conclusions: A stimulus poor in depth cues inaccurately stimulates accommodation in Badal optometers; however, accommodation can be significantly improved in the same Badal optometer, when displaying a realistic image rich in peripheral depth cues, even though these peripheral cues (also referred to as retinal blur cues) are shown in the same plane as the fixation target. These results have important implications in stereoscopic virtual reality systems that fail to represent appropriately retinal blur.Preprin

Suitability of open-field autorefractors as pupillometers and instrument design effects

AIM: To determine the agreement and repeatability of the pupil measurement obtained with VIP-200 (Neuroptics), PowerRef II (Plusoptix), WAM-5500 (Grand Seiko) and study the effects of instrument design on pupillometry. METHODS: Forty patients were measured twice in low, mid and high mesopic. Repeatability was analyzed with the within-subject standard deviation (Sw) and paired t-tests. Agreement was studied with Bland-Altman plots and repeated measures ANOVA. Instrument design analysis consisted on measuring pupil size with PowerRef II simulating monocular and binocular conditions as well as with proximity cues and without proximity cues. RESULTS: The mean difference (standard deviation) between test-retest for low, mid and high mesopic conditions were, respectively: -0.09 (+/- 0.16), -0.05 (+/- 0.18) and -0.08 (+/- 0.23) mm for Neuroptics, -0.05 (+/- 0.17), -0.12 (+/- 0.23) and -0.17 (+/- 0.34) mm for WAM-5500, -0.04 (+/- 0.27), -0.13 (+/- 0.37) and -0.11 (+/- 0.28) mm for PowerRef II. Regarding agreement with Neuroptics, the mean difference for low, mid and high mesopic conditions were, respectively: -0.48 (+/- 0.35), -0.83 (+/- 0.52) and -0.38 (+/- 0.56) mm for WAM-5500, -0.28 (+/- 0.56),-0.70 (+/- 0.55) and -0.61 (+/- 0.54) mm for PowerRef II. The mean difference of binocular minus monocular pupil measurements was: -0.83 (+/- 0.87) mm; and with proximity cues minus without proximity cues was: -0.30 (+/- 0.77) mm. CONCLUSION: All the instruments show similar repeatability. In all illumination conditions, agreement of Neuroptics with WAM-5500 and PowerRef II is not good enough, which can be partially induced due to their open field design.Postprint (published version